In the 1930s on the way to increasing speed, a new phenomenon known as “Flutter” appeared. Many aeronautical researchers dedicated themselves to investigating this phenomenon and the ways to combat it. In the USSR, the research carried out in this field at the TsAGI by a group of specialists headed by Victor Belyayev had a special impact.

In the course of the research process, it was linked to the increase in wing resistance through the change of its shape, achieving a self-balancing state capable of counteracting the appearance of the “Flutter” throughout the range of increased speeds. To demonstrate this principle, engineers Victor Nikolayevich Belyayev and Victor Ivanovich Yujárin developed a new wing design at the TsAGI . The plan view depicted an open M with a slim profile and large wingspan.

To verify the effectiveness of this new “elastic” wing, the manufacturers, in addition to the tests in the wind tunnel, decided to build two experimental gliders that were successfully tested, allowing to collect valuable information on the way to design a high-speed aircraft with this wing plant.

In 1938, Víctor Belyayev’s group designed an experimental aircraft that received the UK name according to the acronym of “U prugoye K rylo” or Elastic Wing with the new wing structure, which at the time was popularly known as “butterfly” type wing.

The UK design was compact enough to be able to use the lightweight 430 hp Renault Ro.1 12-cylinder engine. In 1936 a group of Soviet specialists had traveled to France to familiarize themselves with this technology and to buy several licenses for the production of Renault engines for the development of light aviation in the USSR. Among these engines was the Ro.1, which was to be produced at Factory No.26 in Ribinsk.

An 11 m² wingspan “butterfly” wing was attached to the fuselage, with a profile of variable thickness throughout the wingspan, 7.7% at the root and 5% at the ends, which was attached to the fuselage in low implantation. Long flaps were located on the trailing edge to improve takeoff and landing characteristics. The thin double-section ailerons were located along the entire span of the positive-taper section of the wing.

The UK was conceived to carry out all the high-school piloting maneuvers on it and was equipped with special recording equipment to measure the deformations of the wing and the impact on the stability of the aircraft in the different flight regimes. The structure was calculated to withstand overloads of 13 units.

The construction of both the wings and the fuselage was made of wood.

The landing gear was of the conventional retractable type, with a small tail wheel and the main landers that were collected into elliptical cavities located in the fusion zone of the fuselage and the wing root.

The cockpit was closed, with capacity for two crew members in tandem: the pilot and a specialist, destined to guarantee the correct registration of the instruments.

The fuel system included tanks located in the wing, in the negative sagging zone.

A single-seater version known as UK-1 was also conceived designed as a sports aircraft with the aim of setting a new speed record in land aircraft.

The excellent results obtained during the tests with the Belyayev gliders, as well as the excellent results that the project calculations yielded, led the TsAGI management to “think big”, so it was decided to close the development of the record-breaking aircraft and concentrate on designing a much more ambitious project: the DB-LK bomber, with an inverted swept wing similar to that used by gliders. This bomber was designed in 1938 and in its state tests, which lasted until 1941, it showed better performance than the Ilyushin DB-3M with the same power plant and bomb load.

Despite the reorientation of the project, the idea of building the single-engine model with an “elastic” wing was not rejected. At the request of the TsAGI the OKB for light constructions of the Kazan Aviation Institute (KAI) , under the direction of ZI Itskplich (who until 1939 worked on the OKB-301 dedicated to the study of Caudron models in the USSR under the direction of AA Dubrovin ) and later by GN Borobiov, two specimens were studied: the UK-1A with the “butterfly” wing and the UK-1B with a low wing of normal configuration. Both were two-seater and differed only in the wing, in order to be able to carry out comparative experiments.

Due to the fact that the Renault Ro-1 engine was never produced, it was decided to install the MV-6 engine (a copy of the 6-cylinder Renault Bengali 6 produced under license in the USSR) in both examples, which was characterized by having only half the projected power. This reality definitively removed the idea of being able to establish a new speed record, but despite this it was thought that, in parallel with the experimental tasks, the aircraft could be ideal for participating in different air competitions.

The first of two aircraft built at KAI was completed in 1940. In the spring of the following year it was handed over to the TsAGI for the development of the tests. With the beginning of the war these were not realized.

The second was never finished. Actual flight data was not preserved.

Engine: A 430 hp Renault Ro.1 12-cylinder engine
Wingspan: 10.8 m
Wing area: 11.0 sq.m
Length: 6.84 m
Tailplane wingspan: 2.2 m
Empty weight: 750 m
Takeoff weight: 1028 m
Speed at altitude: 510 km / h
Landing speed: 100 km / h
Ceiling: 11000 m
Range: 900 km
Accommodation: 2

The Colibri was built in 1937 by Quinto Beltrame. It is plywood monocoque, with an 18 hp opposed-piston single cylinder air cooled two-stroke Beltrame engine, and three drive belts to the prop. The 450 cc engine produces 18 hp at 4200 RPM, with oil and fuel mixed.

The elevator is the only controllable surface: +12.5 degrees, – 2.5 degrees, 30 degrees lateral tilt each side.

Engine: Beltrame 450 cc, 18 hp
Wingspan: 6.06 m
Length: 3.55 m
Height: 1 m
Wing area: .5 sq m
Empty weight: 70 kg
Loaded weight: 160 kg
Stall speed: 60 km/h
Cruise speed: 140 km/h
Max speed: 160 km/h
Take off run: 125 m
Landing roll: 50 m
Range: 500 km